1. Field of the Invention
The present invention relates to crystal growth apparatus for growing a semiconductor crystal on a substrate in the liquid phase within a reactor which has a temperature gradient along its axis.
2. Description of the Prior Art
Needless to say, it is the most important in the semi-conductor industry to fabricate semiconductor elements of good characteristics at a high yield. As regards the manufacturing step of epitaxially growing a semiconductor crystal, this is equivalent to precisely bestowing uniform thicknesses, carrier impurity concentration profiles, composition profiles etc. on all the parts of a layer epitaxially grown on a substrate crystal.
It is well known that the most serious cause which brings a dispersion or variation to the grown layer in the liquid phase epitaxial growth of the semiconductor crystal is non-uniformity in the temperature within the plane of the substrate. In, for example, the liquid phase epitaxial growth of GA.sub.(1-x) Al.sub.x As (where x denotes the mixed ratio and 0&lt;x&lt;1), the non-uniformity in the substrate temperature gives rise to non-uniformity in the thickness of the grown layer and also to non-uniformity in the mixed ratio x. Where the material is used for LED's (light emitting diodes), the emission wavelength is dispersed among the diodes.
As another example, where it is intended to obtain by the liquid phase epitaxial growth of GaAs a multi-layer which has layers of different impurity concentration levels, the non-uniformity in the substrate temperature gives rise to non-uniformity in the thickness of each constituent layer and also to non-uniformity in the impurity concentration. Both these non-uniformities impart dispersions or variations in the characteristics of the semiconductor elements, and lower the yield of the fabrication of the elements.
As understood from the above explanation, to the end of fabricating the semiconductor elements in a good yield, it is very important to eliminate the non-uniformity in the substrate temperature in case of employing epitaxial growth.
The non-uniformity in the temperature within the substrate plane as stated above is generated by various causes. One of them is an arrangement of the substrate that ignores the effect of the shape of a jig. An example of such arrangement will now be described in connection with an apparatus of the so-called "vertical type" among growth apparatus of the sliding type. The apparatus comprises a quartz reactor which is assembled in the middle of an electric furnace shaped as a hollow cylinder, and a graphite jig which is set in the quartz reactor. The upper part of the graphite jig is a slider portion which, having received a solution therein, is rotated by a rod so that it can bring the substrate and the solution into contact with each other. On the other hand, the lower part of the jig is a fixed portion which holds the substrate and which is fixed by a tube made of quartz glass. In such apparatus, it is common knowledge to arrange the substrate so that its surface may become perpendicular to the axis of the reactor. The temperature within the jig in such apparatus, however, is so distributed that the temperature is high at the outside surface of the jig close to the furnace wall and so that the temperature becomes lower in coming nearer to the central part of the jig. For this reason, the temperature within the substrate plane is naturally distributed in a non-uniform manner.
In the furnace which has a uniform temperature along its axis, isothermal planes will be represented by concentric cylinders which have substantially the same axis. Also, in the reactor which has a temperature gradient along its axis, as regards the temperature distribution in one plane perpendicular to the axis, the temperature is high at the outside surface of the jig close to the furnace wall, and it becomes lower as the central part of the jig is approached. Isothermal planes within the jig in that case are upwardly convex. It is therefore natural that where the substrate is arranged so as to become perpendicular to the axis of the reactor (or the axis of the jig), the non-uniformity in the temperature arises within the plane of the substrate.
The specific dispersions or variations in the grown layer which are, as previously stated, ascribable to the non-uniformity in the temperature are particularly intense in case of the epitaxial growth which requires a thickness as large as several hundreds .mu.m for the grown layer, e.g. 200 .mu.m. Due to influences of these dispersions, the yield in the fabrication of the semiconductor elements becomes conspicuously inferior.
As a second example of the arrangement of the substrates which lead to the generation of the non-uniformity in the substrate temperature due to the effect of the shape of a growing jig, description will now be made of an apparatus of the so-called "lateral type" among the growth apparatus of the sliding type. The apparatus comprises a square reactor of quartz glass which is assembled in an electric furnace, and a growing jig of graphite which is set in the reactor. The upper part of the graphite jig is a slider portion which, having received therein a solution for the growth, is moved by a rod. The lower part of the jig is a fixed portion which holds the substrates.
In such apparatus, it is well known that the substrate is arranged so as to become parallel to the center plane of the reactor, that is, the plane of the contact between the slider and the fixed portion. Where the jig is heated by the use of the electric furnace, non-uniformity in the temperature within the jig as explained in the first example of a growth apparatus arises also in the present example. Where a temperature gradient is established in the vertical direction of the jig, non-uniformity in the temperature arises likewise.
Where the substrates are arranged in parallel with the horizontal axis of the reactor in such apparatus, the non-uniformity in the temperature within the substrate plane is naturally caused in many positions. Also, in this case, the characteristics of the grown layer are dispersed or varied by the non-uniformity in the temperature, and hence, the yield in the fabrication of the elements is degraded.